JP2005292245A - Optical element and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem wherein a conventional optical waveguide element needs to be provided with an intermediate layer, when it is formed by sticking together substrates having nearly equal refractive indexes, and then it becomes expensive. <P>SOLUTION: An optical element formed, by directly jointing an optically polished 1st substrate and an optically polished 2nd substrate 2 together has an optical waveguide 5 in the 1st substrate, and has fine voids 3 on a directly jointed surface 4 at least below the optical waveguide 5. Consequently, no intermediate layer is necessary, and the optical element becomes low-priced and is improved in the jointing strength. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical element formed on a directly bonded substrate and a manufacturing method thereof.

Conventional direct bonding techniques are known as techniques for firmly bonding substrates without using an adhesive or the like, and various materials such as glass, semiconductors, ferroelectrics, and piezoelectric ceramics can be bonded with high accuracy. Such a directly bonded substrate can be used as an optical waveguide by, for example, thinning one of the directly bonded substrates (a set of two substrates) and then using it as an optical waveguide, for example. Is expected. So far, an optical waveguide type element has been proposed as an example of an optical element in a direct bonding substrate such as a dielectric substrate, a semiconductor substrate, or a glass substrate. In particular, Patent Document 1 proposes a directly bonded substrate through a thin film as a method of directly bonding lithium niobate or lithium tantalate, which are the above-described ferroelectric crystal substrates, to the same type substrate or glass substrate to form an optical waveguide. . FIG. 3 shows a cross-sectional view of a directly bonded substrate through a thin film disclosed in Patent Document 1. As shown in FIG. 3, the holding substrate 21, the glass substrate 22, the optical waveguide portion 23, and a thin film layer SiO ₂ or SiN 24 are included.
JP-A-6-222229

However, in the conventional configuration, it is difficult to form an optical waveguide on a directly bonded substrate of the same type having the same refractive index. In addition, even when two types of substrates having different refractive indices are directly joined, such as direct joining of a lithium niobate substrate and an Mg-doped lithium niobate substrate, it is possible to form an optical waveguide on a substrate having a smaller refractive index. It was impossible. Since the Japanese Patent Laid-Open No. 6-222229 difficult surface roughness control of the thin film layer, for example, the formation of the SiO ₂ film in the sputtering film or a vapor deposition film is unsuitable for large direct bonding the film surface roughness, for example There has been a problem that an expensive and large-scale apparatus such as a CVD (Chemical Vapor Deposition) apparatus is required. In addition, there is a problem in that the adhesiveness and bonding strength between the thin film and the substrate are unevenly distributed depending on the shape conditions of the thin film, and sufficient strength against machining cannot be obtained.

  The present invention solves the above-described conventional problems, and enables an optical waveguide to be directly formed on a bonded substrate, thereby eliminating the need for an intermediate layer, and providing an optical element having low cost and bonding strength. For the purpose.

  In order to achieve the above object, in an optical element formed by directly joining an optically polished first substrate and an optically polished second substrate, an optical waveguide is provided in the first substrate, and at least the optical waveguide is provided. A minute hole is provided in the direct joint surface under the waveguide.

  According to the present invention, an optical waveguide can be directly formed on a bonded substrate regardless of the refractive index, and an optical element having low cost and bonding strength can be obtained.

(Embodiment 1)
Hereinafter, the optical element according to Embodiment 1 of the present invention will be described with reference to the drawings.

  FIG. 1 is a sectional view of an optical element according to Embodiment 1 of the present invention.

  In FIG. 1, a first substrate 1 made of Mg-doped lithium niobate having a refractive index of 2.19 (in the case of a wavelength of 0.633 μm) and lithium niobate having a refractive index of 2.20 (in the case of a wavelength of 0.633 μm). The second substrate 2 is directly bonded to the direct bonding surface 4, and the minute holes 3 are formed on the direct bonding surface under the optical waveguide 5 formed into a ridge shape by processing the first substrate 1. Is provided. Here, the height of the holes 3 is about 0.5 μm, the diameter is about 0.3 μm, and the volume density is about 0.5. The optical waveguide 5 has a height (height from the hole 3 to the top of the ridge) of about 4 μm, a width of the top of the ridge of about 5 μm, and a ridge height (height from the periphery of the ridge to the top of the ridge). The nodging portion 6 is formed with a depth of about 0.1 μm deeper than the periphery of the ridge portion on the side surface of the ridge.

  By providing the holes 3 in the first substrate 1, it is possible to reduce the refractive index of the region including the holes 3, thereby confining the light in the optical waveguide 5.

  In the first embodiment, the holes 3 are provided in the first substrate 1, but they may be provided in the second substrate 2. However, when the refractive index of the first substrate 1 is smaller than the refractive index of the second substrate 2 as in the first embodiment, it is preferable to provide the first substrate 1 on the first substrate 1. This is desirable because it is easy to increase the difference from the refractive index.

  Even when the refractive index of the first substrate 1 is larger than the refractive index of the second substrate 2, if the difference is small, confinement becomes insufficient, and the effect of the present invention can be obtained.

  Further, if the hole 3 is provided under the optical waveguide 5, the effect of the present invention can be obtained, but it may be provided in the periphery or directly on the entire bonding surface 4. In this case, the optical waveguide 5 and the hole 3 can be easily aligned.

  Next, the shape of the holes will be described. It is possible to form a fine hole in the lower part of the optical waveguide to form a portion having a different refractive index, thereby realizing light confinement. However, it is necessary to take a structure in which the size of the fine structure is sufficiently small with respect to the light propagating through the optical waveguide. When the shape of the fine structure is large, light propagating through the waveguide is scattered by the fine structure, and the loss of the waveguide increases. As the structure, the minute limit of the fine structure is determined by processing accuracy, but the diameter of the hole is preferably 1/2 or less of the wavelength of light propagating through the waveguide, preferably 1/10 or less. When the structure is sufficiently small with respect to the wavelength of light, the substantial refractive index can be controlled by the holes. When fine pores are formed, it is necessary to consider the influence of these distributions on the optical waveguide. When the holes are regularly formed, periodic perturbation is given to the light propagating through the optical waveguide, and light diffraction occurs. This may increase the propagation loss of the optical waveguide. In order to avoid this, it is desirable that the holes be randomly arranged and have no regularity.

  In addition, the refractive index of the portion where the void is formed with the volume density of the void being V and the refractive index of the substrate being n is approximately represented by n * (1−V) + V, and is effective depending on the density of the void. The refractive index can be controlled. Thereby, confinement of the optical waveguide can be controlled. For example, the refractive index of the cladding portion of the optical waveguide can be controlled by partially changing the density of the holes. Furthermore, since the thickness of the cladding can be controlled by changing the depth of the holes, the shape of the waveguide can be freely controlled. By using this structure, there is an advantage that the shape of the waveguide can be controlled three-dimensionally. Application to a mode converter or the like that changes the shape of the taper structure or waveguide mode becomes possible.

  The height of the holes is preferably 1/5 or more of the wavelength of light propagating through the waveguide. The portion where the holes are formed functions as a clad layer having a different refractive index. If this cladding layer is not sufficiently thick, the electric field distribution of light propagating through the optical waveguide leaks to the substrate side, causing an increase in propagation loss of the optical waveguide. The thickness of the cladding layer varies depending on the area density of the holes, but it needs to be at least 1/5 of the wavelength of light propagating through the waveguide. More preferably, one half or more of the wavelength of light propagating through the waveguide is required. When the wavelength of light propagating through the waveguide is less than 1/5, the waveguide loss is significantly increased. When the wavelength is larger than 1/2 of the wavelength of light propagating through the waveguide, a low-loss optical waveguide can be realized. In addition, as the height increases, the aspect ratio of the projections or holes increases and the difficulty of processing increases. The height limit is determined by the processing accuracy.

  Next, the arrangement of the holes will be described. There are two methods for arranging the holes, which are also related to the shape of the holes. When holes are provided in the first substrate 1 and used as a clad layer surrounding the waveguide layer, the clad layer needs to be effectively in a state where the refractive index is smaller than that of the wave guide layer. In this case, the holes in the cladding layer need to have a size such that the light propagating through the waveguide layer does not feel the holes as substantial irregularities, that is, the size of the wavelength of the light to be guided is ½ or less. In addition, it is necessary to randomly arrange the holes. This is because when there is periodicity, scattering of guided light by diffraction increases.

  On the other hand, there is a method of confining light using diffraction by holes. It is a method that uses a diffraction effect called photonic crystal and diffracts light using two-dimensional Bragg reflection. In this case, the holes are arranged in order to use diffraction. The arrangement of is preferably a periodic arrangement. By forming periodic vacancies in the bottom surface of the waveguide layer, confinement in the depth direction using light diffraction becomes possible.

  Moreover, although the shape of the holes has shown a two-dimensional structure, a one-dimensional stripe structure is also possible.

  Next, the shape of the ridge portion will be described. When the width of the optical waveguide 5 is less than twice the wavelength of light propagating through the waveguide, the propagation loss of the waveguide is greatly increased. Further, when the waveguide width becomes larger than 10 times the wavelength of light propagating through the waveguide, the light propagating through the waveguide cannot propagate in a single mode. Therefore, the width of the optical waveguide 5 is twice to 10 times the wavelength of the propagating light. Double is preferred.

  As for the height of the optical waveguide 5, the single mode property of the waveguide mode is determined by the height of the optical waveguide and the shape of the ridge, and the height of the optical waveguide is 2 to 5 times the wavelength of light propagating through the waveguide. Thus, a low-loss single mode waveguide can be constructed. When the height of the optical waveguide is less than twice the wavelength of light propagating through the waveguide, the propagation loss increases. If the thickness of the waveguide layer is greater than 5 times the wavelength of light propagating in the waveguide, it becomes difficult to design the waveguide in a single mode, and the height of the optical waveguide is equal to the wavelength of light propagating in the optical waveguide. 2 to 5 times is preferable.

  Next, the nodge shape will be described. The lateral confinement effect of the optical waveguide due to the formation of the nodage portion is particularly effective when the optical waveguide is applied to a wavelength conversion element. In the wavelength conversion element, the fundamental wave is converted into a harmonic in the optical waveguide. The conversion efficiency in this case is determined by the overlap of the electric field distribution of the fundamental wave and the harmonic wave in the waveguide. In a highly confined optical waveguide having a refractive index distribution on the step, the electric field distributions of the fundamental wave and the harmonics are theoretically almost the same, so that high efficiency can be achieved.

  However, in a conventional optical waveguide that does not have a nodge shape, the leakage of guided light is large to the side surface, and the overlapping of the charge distribution of the fundamental wave and the harmonic wave is reduced. On the other hand, by forming the nodge portion, the confinement of the waveguide light in the lateral direction can be dramatically improved, and further the improvement of the power density of the waveguide can be realized. It becomes possible to increase it 5 times. A highly confined optical waveguide structure can be realized by the optical waveguide in which the nodge portion is formed.

  The optical waveguide structure in which the nodge portion is formed is effective not only for the wavelength conversion element but also for an element such as an optical switch using the electro-optic effect. By increasing the lateral confinement of guided light, the difference in propagation constant between the TE mode and the TM mode, which are waveguide propagation modes, can be designed to be almost the same. The characteristic of the optical waveguide by polishing is that both TE and TM modes can be guided in the same manner as the metal diffusion waveguide unlike the proton exchange waveguide. If propagation in both modes is possible, an unpolarized waveguide device indispensable for application to the optical communication field can be realized. However, in order to utilize the non-polarized characteristic, it is necessary to control the propagation constants of the TE and TM modes that propagate through the optical waveguide. In conventional optical waveguides, since the lateral confinement is weak, it is difficult to match the propagation constants of both modes. On the other hand, the formation of a nodge portion enhances the lateral confinement of the optical waveguide. , TE (Transverse Electric), TM (Transverse Magnetic) mode propagation constants can be matched. Furthermore, since the refractive index of the cladding layer can be controlled by the fine holes, it is easier to control the propagation constant between modes.

  In addition, the effect of the nodge formed on the side surface of the ridge portion can improve the mechanical strength of the device as compared with the method of reducing the thickness around the ridge portion.

  The manufacturing method will be described below.

  In order to realize the element structure of the present embodiment, first, a minute recess is formed on the direct bonding surface of the first substrate 1. Although there are various methods for forming a minute recess, dry etching is used in the first embodiment. First, a metal film is formed on the entire surface by sputtering, and a mask pattern is formed using a positive photoresist. For pattern formation, a reduced projection type exposure apparatus using I-line as a light source is used to form a minute pattern, and tilt illumination is used for the illumination system, so that a minute mask pattern of about 0.3 μm can be formed. . Thereafter, the metal film is etched by dry etching using the photoresist as a mask to form a minute mask pattern using the metal film.

  Next, after removing the photoresist used for the mask, the first substrate 1 is etched by dry etching using the metal film as a mask to form minute recesses. An ICP (Inductively Coupled Plasma) type etching apparatus capable of generating high-density plasma in a high vacuum is used in order to process minute concave portions with high accuracy. As a result, the depth variation of the recess can be suppressed to 5% or less with respect to about 0.5 μm. Thereafter, the metal film used for the mask is removed.

  Next, the main surfaces of the first substrate and the second substrate are subjected to a hydrophilic treatment, and the close contact state is realized by pressurization by aligning the crystal axes of the first substrate and the second substrate. Thereafter, the bonded substrate can be directly formed by heat treatment at about 500 ° C.

  In order to form the optical waveguide 5 on the bonded substrate having the minute holes 3 obtained in this way, the thickness of the first substrate is polished to about 4.5 μm. Thereafter, a step having a width of about 5 μm and a width of about 1.5 μm is formed on the first substrate 1 by using photolithography and dry etching.

  As described above, in this embodiment, in the optical element formed by directly bonding the optically polished first substrate 1 and the optically polished second substrate 2, the optical waveguide 5 is provided in the first substrate 1. It is possible to form an optical clad portion having a different refractive index having mechanical strength under the optical waveguide 5 by forming the minute holes 3 in the direct joining surface 4 under the optical waveguide 5 at least. In addition, an optical waveguide can be formed with a direct bonding substrate.

  Further, in order to realize direct bonding, a surface roughness of Rmax 5 nm or less is required, so that 1 μm-sized dust becomes a direct bonding defect factor. In the case where a direct bonding region is formed by minute holes of about 0.3 μm as in the present embodiment, the effect of reducing bonding defects due to dust, which is a problem during manufacturing, can also be realized by this element structure.

  Next, the nodage portion 6 of the optical waveguide 5 will be described in detail. A pattern of the nodge portion 6 is formed by a positive type photoresist. Thereafter, the first substrate 1 is dry-etched using a photoresist as a mask by dry etching, so that the nodule portion 6 and the ridge-type optical waveguide 5 are simultaneously formed. In dry etching, an organic protective film made of a photoresist is formed on the side wall of the optical waveguide 5, and anisotropic etching can be realized by suppressing side etching. Further, the side surface of the ridge optical waveguide 5 can be realized to have an inclined wall shape of 60 to 89 degrees in the depth direction by controlling the effect of the side wall protective film.

  The number of ions that etch the substrate increases at the end of the inclined wall due to the effect of the inclined wall shape. In this way, the nodging portion 6 etched deeper than the periphery of the optical waveguide 5 can be formed on the side surface of the ridge optical waveguide 5. This nodge shape cannot be stably machined by grinding of the grinding wheel edge in grinding using a grinding wheel. In addition, even when laser thermal processing using an excimer laser is used, the energy distribution of the laser spot has a high central energy distribution, so that a nodge shape cannot be realized. Thus, the nodule portion 6 on the side surface of the ridge-type optical waveguide 5 can be processed by ionic dry etching. The light confinement efficiency can be increased by the element structure having the nodule portion 6 using this manufacturing method.

(Embodiment 2)
Hereinafter, a second embodiment of the present invention will be described with reference to the drawings.

  FIG. 2 is a cross-sectional view showing the configuration of the optical element according to Embodiment 2 of the present invention.

  The difference between the second embodiment and the first embodiment is that the optical waveguide is formed by forming a ridge on the first substrate in the first embodiment, but the optical waveguide is formed in the second embodiment. By forming minute concave portions on the side surfaces, a clad layer having a different refractive index is formed, and light can be confined.

  The clad layer formed along the optical waveguide 5 has a structure formed by the fine recesses 7, and an effect of forming a forbidden band (photonics band) for light by periodic fine holes is obtained. Further, the depth of the recessed portions 7 to be arranged is made deeper at the side surface portion of the optical waveguide 5, and the effect of efficiently confining light is obtained by the effect of the deep recessed portions.

  In the second embodiment, the concave portion 7 is formed in the first substrate 1 and the depth of the concave portion in the side surface portion of the optical waveguide 5 is increased, so that the nodification effect of the first embodiment is exhibited. It is what strengthens optical confinement.

  The element structure having the nodge 6 in the first embodiment is that the diameter of the recess 7 is about 0.3 μm, the depth of the recess on the side surface of the optical waveguide 5 is about 1.2 μm, and the peripheral depth is about 1 μm. It is possible to increase the light confinement efficiency in the same manner as the above.

  Further, the formation of the optical waveguide 5 by the concave portion is also effective for enhancing the effect of improving the mechanical strength of the optical element.

  An optical element according to the present invention is an optical element formed by directly joining an optically polished first substrate and an optically polished second substrate, and includes an optical waveguide in the first substrate, at least of the optical waveguide. An optical element with a minute hole in the lower direct bonding surface, and it becomes possible to control the effective refractive index by forming a minute hole in the lower part of the optical waveguide. It can be improved and applied to elements such as a wavelength conversion device, a waveguide mode converter, and an optical switch using the electro-optic effect.

Sectional drawing which shows the structure of the optical element in Embodiment 1 of this invention Sectional drawing which shows the structure of the optical element in Embodiment 2 of this invention Sectional drawing which shows the structure of the optical element in a prior art example

Explanation of symbols

1 first substrate 2 second substrate 3 holes 4 direct junction 5 waveguide 6 Nudge section 7 the recess 21 hold the substrate 22 glass substrate 23 an optical waveguide unit 24 thin layer SiO ₂ SiN

Claims (14)

In an optical element formed by directly bonding an optically polished first substrate and an optically polished second substrate, an optical waveguide is provided in the first substrate, and at least a direct bonding surface under the optical waveguide An optical element with a minute hole. The optical element according to claim 1, wherein the refractive index of the first substrate and the second substrate are substantially equal, or the refractive index of the first substrate is smaller than the refractive index of the second substrate. The optical element according to claim 1, wherein the optical waveguide is a ridge-type optical waveguide. The optical element according to claim 3, wherein the side surface of the ridge portion of the optical waveguide has a nodule shape. The optical element according to claim 1, wherein the clad layer having a different refractive index formed on the side surface of the optical waveguide is formed by a minute recess. 6. The optical element according to claim 5, wherein in the arrangement of the concave portions constituting the clad layers having different refractive indexes formed on the side surface of the optical waveguide, the depth of the concave portion on the side surface of the optical waveguide is deeper than the depth outside the optical waveguide. 2. The optical element according to claim 1, wherein the width of the optical waveguide is 2 to 10 times the wavelength of light propagating through the optical waveguide. The optical element according to claim 1, wherein the height of the layer of the optical waveguide is 2 to 5 times the wavelength of light propagating through the optical waveguide. The optical element according to claim 1, wherein the width of the minute hole is not more than ½ of the wavelength of light propagating through the optical waveguide. 2. The optical element according to claim 1, wherein the height of the minute hole is 1/5 or more of the wavelength of light propagating through the optical waveguide. The optical element according to claim 1, wherein the array of minute holes is not periodic. 2. The optical element according to claim 1, wherein the minute holes have a stripe shape, and the width of the holes is equal to or less than ½ of the wavelength of light propagating through the optical waveguide. After forming a minute recess in at least a part of the direct bonding surface of the first substrate or the second substrate, a minute hole is formed in the direct bonding surface by directly bonding the first substrate and the second substrate. And forming the optical waveguide by processing the first substrate above the holes. 14. The method of manufacturing an optical element according to claim 13, wherein in forming the optical waveguide, a ridge is formed by dry etching using a photoresist as a mask, and at the same time a nodule portion is formed.

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